1. Field of the Invention
The present invention relates in general to the electrographic recording of toner images on a recording medium and more specifically to a means and a method for supplying an optimum amount of toner powder to a recording electrode.
2. Description of the Prior Art
Electrographic recording systems for printing toner images on a recording medium have in common the use of toner applicators for transporting toner to a recording region. Various types of toner transport systems have been devised as illustrated by Fotland et al, U.S. Pat. No. 3,121,375 and Nau et al, No. 2,932,548, which both teach the application of toner into the recording region through what serves as a recording electrode of the systems shown. Such toner applicators may serve satisfactorily in systems employing a relatively small number of recording electrodes; however, in systems having 200-400 recording electrodes per inch the application of toner through each such electrode is undesirable.
A second common method for delivering toner into the recording region is the use of a separate developing element to which toner is first applied. The developing element may be in the form of a porous endless belt on which the toner rides into the recording region as disclosed by Capps, U.S. Pat. No. 3,355,743, or in the form of a cylinder that encloses a magnet rotor, the rotation of which brings the toner into the recording region as taught in Anderson, U.S. Pat. No. 3,455,276. In Anderson, a doctor blade is employed to meter a uniform layer of toner onto the transport cylinder.
A primary deficiency with the above types of prior art toner applicators is that they do not provide sufficiently accurate metering of toner into the recording region when large numbers of narrowly spaced apart electrodes are employed in a printing array. It is difficult to precisely meter thin, uniform amounts of toner powder into the recording region without experiencing occasional plugging of metering orifices or doctor blade gaps because of toner powder agglomeration. If too much toner is transported into the recording region the toner images formed will tend to bloom out and will not have sharply defined edges. Also, when using plastically deformable toners such as thermoplastic resin, an excessive amount of toner in the recording region may increase pressure applied on the toner by the recording electrode and the recording medium to the point that there is a gradual toner build up on the recording electrode due to pressure fusing. If inadequate toner is transported onto the recording region, the formed toner images suffer a loss of density and become light or disappear altogether.
The present invention provides a toner applicator designed to overcome the deficiencies of the above described systems by accurately metering an optimum level of toner into the recording region in order that high quality, high resolution toner images may be formed.